How Smallpox Reached the Americas: First Genetic Evidence Points to Europeans

More than 400 years ago, two people living near the edge of the Inca Empire died while sick with smallpox. Scientists studied the viral DNA found in their preserved remains, which were discovered in northern Chile. The findings give us the first genetic proof of how and when smallpox arrived in the Americas. A new study published in the journal Science confirms that European colonizers brought the disease to the region.

Smallpox was one of the deadliest diseases in all of human history. The virus killed an estimated 300 million people during the twentieth century alone. When smallpox reached the Americas, it was equally devastating, killing millions of people across two continents. Scientists had long suspected that European explorers and settlers spread the disease, but solid evidence had been hard to find.

The reason is that viruses do not leave much behind in the archaeological record. Until now, researchers had found only two examples of smallpox DNA from before the twentieth century. One came from Viking remains about 1,400 years old, and the other came from a mummy found in Lithuania from the 1600s. The limited evidence already showed that the smallpox virus changed greatly over many centuries.

This latest discovery was actually made by accident. The research team was originally using the Chilean mummy remains to test DNA analysis software, hoping to learn about human history at the site. But when they ran the DNA samples through the software, it picked up traces of an unknown smallpox variant — a version of the virus no one had recorded before. The team was completely surprised by what they found.

Co-author Constanza de la Fuente Castro, a biological anthropologist at the University of Chile, said finding the virus was unexpected. She said it 'didn't cross my mind,' but that 'makes it even more exciting.' Her team compared the new variant to other ancient and modern smallpox genomes to figure out where it came from. The comparison pointed clearly to a European origin.

The Chilean variant appeared to split off from a European branch of the smallpox virus around the thirteenth century. This strongly supports the idea that European colonizers brought smallpox to South America. The study also adds to growing evidence that the smallpox virus lost genes over time as it adapted to human hosts. Terry Jones, a virologist in Berlin, said studies like this give scientists a rare look into a pathogen's past.